Parts of the South could see thunderstorms and periods of heavy rain this afternoon.
MetService said there was a warm, moist air mass covering the country, with cooler air above. This was creating a very unstable atmosphere, especially over the North Island, where heavy showers and thunderstorms were likely in many areas.
A severe thunderstorm watch for localised downpours has been issued for some North Island areas.
The forecaster said thunderstorms were also possible in the South Island, about the Canterbury High Country and inland Otago from the afternoon.
There was a greater chance of storms for parts of Fiordland and Southland, and if they occurred they were likely to produce localised heavy rain.
MetService meteorologist Peter Little commented on the chance of storms earlier this week.
"A ridge of high pressure over the country this week brings light winds to many places, which will allow sea breezes to develop during the daytime. Where these sea breezes converge with light winds from another direction it will force the air to be directed upwards, resulting in cloud build-ups, showers and potentially thunderstorms.
"Converging winds alone don't always lead to the development of showers and thunderstorms. There also needs to be enough warmth and moisture near the surface, and cooler than normal temperatures in the upper atmosphere,which makes conditions unstable."
